JPL Navigators Drive Two-for-one Comet Mission

NASA's Comet Nucleus Tour, slated to launch no earlier than July 3, will
rely on the Jet Propulsion Laboratory's navigation experts to guide the craft on its
tricky journey toward two comets to find out how the icy, rocky bodies evolve as they
approach the Sun.

The spacecraft is slated for a 15-month journey to Comet Encke followed by a
two-and-a-half-year trip to Comet Schwassmann-Wachmann 3. The mission was
conceived so that scientists could compare the older, less active Encke to the younger,
dust-clouded Schwassmann-Wachmann 3. The different targets pose a challenge to
the navigators, too.

"We'll be flying by quickly and close to Comet Encke. There will be just ten
minutes of time to take the science data, and our job is to protect that time," said
Tony Taylor, chief of the navigation team at JPL, in Pasadena, Calif. "On the other
hand, Comet Schwassman-Wachmann 3 has more dust and gas shooting from its
inner body. We will fly past it a bit farther away to avoid being hit by a particularly
large particle, and we'll have more time to observe the comet."

The navigation team will guide the spacecraft through its complex orbit. The
cleverly developed launch plan will first send the spacecraft into an Earth-circling
orbit. After six weeks, the navigators will steer the spacecraft toward the first of the
two comets.

"It's like having two launches," said Dr. Bobby Williams, a member of the
navigation team and the leader of the JPL navigation team that landed the Near Earth
Asteroid Rendezvous spacecraft on the asteroid Eros in February 2001. "We have to
fire a rocket to go into orbit around Earth and then about six weeks later fire another
rocket to push the spacecraft out of Earth orbit."

The spacecraft will fly by each comet at the peak of its activity as it
approaches the Sun. During each encounter, the target comet will be well situated in
the night sky for astronomers worldwide to make concurrent observations from the
ground. Protected by its dust shield, the spacecraft will fly by each comet nucleus to
within a distance of 100 kilometers (62 miles). The most intensive data taking will
occur within a day or so of each encounter.

The mission's design is flexible so that the spacecraft can be retargeted to
intercept an unexpected comet visitor. If a "new" comet passes close enough to
Earth's orbit, mission managers at the Johns Hopkins University Applied Physics
Laboratory, Laurel, Md., will design a new flight path to take advantage of the
opportunity to study the new comet. The JPL navigation team will then calculate the
amount of fuel the spacecraft should burn, and for how long, to put it on the right
path.

JPL will also provide communications support through the Deep Space
Network, the worldwide series of antennas that provide radio communications for all
of NASA's interplanetary spacecraft.

"JPL's participation is essential to making the mission happen," said Dr.
Joseph Veverka, principal investigator and leader of the mission from Cornell
University, Ithaca, N.Y. "We have to get the spacecraft very close to the comets and
we have to communicate with the spacecraft – and we couldn't do those things
without JPL. And one of the world's experts on comets, Dr. Don Yeomans of JPL, is
part of our science team."

Comets may have brought to the forming Earth some of the water in the
oceans, some of the gases of our atmosphere and perhaps even the building blocks
from which life arose.

JPL is managed for NASA by the California Institute of Technology,
Pasadena, Calif. The Johns Hopkins University Applied Physics Laboratory manages
the mission, built the spacecraft and its two cameras and will operate the spacecraft
during flight. NASA's Goddard Space Flight Center, Greenbelt, Md., provided the
spacecraft's neutral gas/ion mass spectrometer. Von Hoerner & Sulger, GmbH,
Schwetzingen, Germany, built the dust analyzer. Veverka leads a science team of 18
co-investigators from universities, industry and government agencies in the United
States and Europe. More information on the mission is available at
http://www.contour2002.org